1. Field of the Invention
The present invention relates to a calibration method to calibrate a lithographic apparatus. Further, the invention relates to a lithographic apparatus including a control system to calibrate the apparatus, as well as to a computer product to perform a calibration method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes a substrate table. A position of the substrate table is measured by a measurement system including position sensors. The position sensors may include, e.g., an interferometer and/or an encoder. The interferometer is a type of optical measurement device which measures a distance towards a reference, commonly a mirror or other reflecting surface. The encoder is a type of optical measurement device which measures a position of a reference by detecting an interaction of an optical beam with a pattern on the reference, the pattern including a grid, grating, etc. Depending on a position of the reference, a different reflection, diffraction, etc. will occur, which is detected and thus measured, thereby providing position information regarding the reference.
In an existing design of the lithographic apparatus, the substrate table includes reflecting sides. The interferometers use these reflecting sides as mirrors by directing a measurement beam thereto. A plurality of the interferometers is used directed at different sides of the substrate table. Preferably, at least two interferometers are directed to a same reflecting side of the substrate table, thereby providing information as to a length of an optical part between the interferometer and the reflecting side of the substrate table. Thereby, a position of the substrate table may be measured in a plane substantially parallel to the surface of the substrate held by the substrate table. This plane is commonly indicated as the X, Y plane, while the dimension perpendicular thereto is referred to as the Z dimension. By such a combination of interferometers, a position of the substrate table may be provided in X and Y direction as well as in a rotation with respect to the Z axis.
However, the reflecting sides have imperfections. In particular, they may show irregularities in flatness on the surface thereof, which irregularities are position dependent and result in an error in the measurement of the position of the substrate table. This error is also dependent on the position at which the beam of the interferometer is reflected by the reflective side. For example, an error of the interferometer to measure a position of the substrate table in X direction is dependant on a position in Y direction of the substrate table, and vice versa. An error in a measurement of a rotation of the substrate table with respect to the Z axis is dependant on a position of the substrate table in Y direction, assuming that the rotation is measured by directing two or more interferometers towards a side of the substrate table which is parallel to the Y direction.
In order to compensate for these errors, it is known to perform a calibration process, in which—a pattern is repeatedly irradiated onto the surface of the substrate, the substrate being displaced between successive irradiations thereby irradiating the patterns next to each other, or partly overlapping, to form a single dimensional arrangement of patterns on the surface of the substrate, the arrangement extending preferably in X or Y direction. The patterns are read out and incremental position deviations are derived from reading out neighboring (e.g. overlapping) patterns. Therefrom, a position error is derived, which may be used to calibrate the position of the substrate table in the dimension in question.
It is noted that this calibration does not provide a separate calibration for the measurement system, but provides a calibration of the positioning system as a whole which provides for the positioning of the substrate table, the measurement system forming part of that positioning system.
In recent designs, requirements as to the accuracy of the position measurement system of the substrate table are increased, and use has been made of a two dimensional grating provided over the substrate table. The substrate table is provided with a plurality of sensors, which provide position information by directing appropriate measurement beams towards the grid or grating. The measurement sensors may, e.g., include interferometers, encoders, or any combination thereof, depending on the position information to be obtained from that particular sensor. In these configurations, each of the sensors is prone to an error, which is dependant on the position of the substrate table in both X and Y directions. This is caused by the fact that irregularities of these grating plates also result in local position errors. From the signals provided by the individual sensors, a measured position of the substrate table in each of these degrees of freedom thus also shows a position dependent error (deviation from the actual position of the table).
Consequently, here also a calibration process is desired to compensate for the effects of imperfections in the measurement system.